Add issue templates to GitLab

Problem: We need common grounds for how to write tickets to cover all important aspects per type of ticket, so we have a unified ticket format.

Goal: Finalise and transfer the ticket templates to GitLab so they can be used natively.

ACs:

• Finalise the ticket templates
• Transfer them to GitLab
  • convert the formatting into Markdown equivalent
  • place in the appropriate location of the planning repo (https://docs.gitlab.com/user/project/description_templates/)

Notes/Resources:

• compare https://git.fitko.de/fit-connect/planning/-/tree/main/.gitlab/issue_templates
• drafts for our issue templates: https://nextcloud.fitko.de/f/2072529 (according to the team agreement)

Who needs to be involved / informed

• involved: REs
• informed: the whole team (everyone who potentially will create tickets)